lib.strings: add trim

[MattSturgeon]

strings.trim returns a copy of the string with all leading and trailing whitespace removed.

strings.trimWith does the same thing, but calling code can decide whether to trim the start and/or end of the string.

The obvious regex [[:string:]]*(.*)[[:string:]]* wouldn't work, because the (.*) is greedy and matches trailing whitespace. Since ERE doesn't support lazy/reluctant matches (.*?) introduced in perl-style regexes, we need to use a regex that fails to match blank/empty strings. This case is handled using optionalString.

Description of changes

trim is a fairly core function included in most standard libraries. I've personally come across a couple scenarios where I would've liked to have been able to trim strings.

I've added a few basic tests.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

Pinging recent contributors to lib.string: @infinisil @AndersonTorres @FireyFly @eclairevoyant @h7x4

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[eclairevoyant]

Should this be added to the top-level (lib/default.nix) as well? (Example PR: #214021)

[MattSturgeon]

Should this be added to the top-level (lib/default.nix) as well? (Example PR: #214021)

Perhaps, but based on #266103 I'd guess it's preferred not to add new functions to the top-level?

[infinisil]

I'll try to review this a bit more closely soon, but yeah for now it should still be exported under lib.* too, because that's the status-quo :)

changes addressed

[MattSturgeon]

Would nixpkgs be open to also having trimStart and trimEnd (possible aliases/alternate names trimHead/trimTail, trimLeading/trimTrailing)?

If so, would you want them in this PR or separately?

[AndersonTorres]

What is the prevalence of trim to begin with?

In the sense of: "all these lines of code in Nixpkgs are mere applications of trim".

[eclairevoyant]

What is the prevalence of trim to begin with?

In the sense of: "all these lines of code in Nixpkgs are mere applications of trim".

nixpkgs/lib/strings.nix

Line 1060 in 4eb44db

# RegEx: Match any leading whitespace, possibly a '-', one or more digits,

toInt and friends all try to implement trim

nixpkgs/lib/strings.nix

Line 1174 in 4eb44db

fileContents = file: removeSuffix "\n" (readFile file);

is more of a trimEnd, but still

Also, I'd want to trim option descriptions in some cases (especially for mkEnableOption).

changes addressed

[MattSturgeon]

As suggested (#315411 (comment)) I've implemented a configurable trim' trimWith too.

@AndersonTorres does this achieve what you wanted?

@eclairevoyant would you mind re-reviewing since the implementation has changed?

[infinisil]

Just a minor comment, otherwise I think this is good!

[infinisil]

Looking good to me, nice work!

[infinisil]

Small follow-up: #330034

[github-actions]

Successfully created backport PR for release-24.05:

• [Backport release-24.05] lib.strings: add trim #354227